Solvation-Guided Design of Fluorescent Probes for Discrimination of Amyloids.

The deposition of insoluble protein aggregates in the brain is a hallmark of many neurodegenerative diseases. While their exact role in neurodegeneration remains unclear, the presence of these amyloid deposits often precedes clinical symptoms. As a result, recent progress in imaging methods that utilize amyloid-specific small molecule probes have become a promising avenue for antemortem disease diagnosis. Here, we present a series of amino-aryl cyanoacrylate (AACA) fluorophores that show a turn-on fluorescence signal upon binding to amyloids in solution and in tissue. Using a theoretical model for environmental sensitivity of fluorescence together with ab initio computational modeling of the effects of polar environment on electron density distribution and conformational dynamics, we designed, synthesized, and evaluated a set of fluorophores that (1) bind to aggregated forms of Alzheimer's-related ß-amyloid peptides with low micromolar to high nanomolar affinities and (2) have the capability to fluorescently discriminate different amyloids based on differences in amino acid composition within the binding pocket through exploitation of their solvatochromic properties. These studies showcase the rational design of a family of amyloid-binding imaging agents that could be integrated with new optical approaches for the clinical diagnosis of amyloidoses, where accurate identification of the specific neurodegenerative disease could aid in the selection of a proper course for treatment.

Apparent shear sensitivity of molecular rotors in various solvents.

Fluorescent environment-sensitive dyes often change their spectral properties concomitantly with multiple solvent properties, such as polarity, protonation, hydrogen bond formation, or viscosity. Careful consideration of the response is needed when a fluorescent dye is used to report a single property. Recently, we observed an increase of emission intensity of viscosity-sensitive molecular rotors in fluids subject to flow and speculated that either polar-polar interaction or hydrogen bond formation play a role in the apparent flow sensitivity. In this study, we show experimental evidence that photoisomerization to an isomer with a lower quantum yield, first proposed by Rumble et al. (J Phys Chem A 116(44):10786-10792, 2012), plays a key role in the observed phenomenon. We subjected four molecular rotors with different electron acceptor motifs to fluid flow in solvents of different polarity and ability to form hydrogen bonds. We also measured the isomerization dynamics in a custom fluorophotometer with extremely low light exposure. Our results indicate that the photoisomerization rate depends both on the solvent and on the electron acceptor group, as does the recovery of the original isomer in the dark. In most solvents, recovery of the dark isomer is much more rapid than originally reported, and a state of quasi-equilibrium between both isomers is possible. Moreover, the sensitivity (i.e., relative intensity increase at the same flow rate) is also solvent-dependent. The intensity increase can be detected at very low velocities (as low as 0.06 mm/s). Characteristic for fluorescent dyes is the high spatial resolution, and no flow measurement device with comparable sensitivity and spatial resolution exists, although the nature of the solvent needs to be taken into account for quantitative flow measurement.

Cyclopenta[b]naphthalene cyanoacrylate dyes: synthesis and evaluation as fluorescent molecular rotors.

We describe the design, synthesis and fluorescent profile of a family of environment-sensitive dyes in which a dimethylamino (donor) group is conjugated to a cyanoacrylate (acceptor) unit via a cyclopenta[b]naphthalene ring system. This assembly satisfies the typical D-π-A motif of a fluorescent molecular rotor and exhibits solvatochromic and viscosity-sensitive fluorescence emission. The central naphthalene ring system of these dyes was synthesized via a novel intramolecular dehydrogenative dehydro-Diels-Alder (IDDDA) reaction that permits incorporation of the donor and acceptor groups in variable positions around the aromatic core. A bathochromic shift of excitation and emission peaks was observed with increasing solvent polarity but the dyes exhibited a complex emission pattern with a second red emission band when dissolved in nonpolar solvents. Consistent with other known molecular rotors, the emission intensity increased with increasing viscosity. Interestingly, closer spatial proximity between the donor and the acceptor groups led to decreased viscosity sensitivity combined with an increased quantum yield. This observation indicates that structural hindrance of intramolecular rotation dominates when the donor and acceptor groups are in close proximity. The examined compounds give insight into how excited state intramolecular rotation can be influenced by both the solvent and the chemical structure.

A-ring oxygenation modulates the chemistry and bioactivity of caged Garcinia xanthones.

Natural products of the caged Garcinia xanthones (CGX) family are characterized by a unique chemical structure, potent bioactivities and promising pharmacological profiles. We have developed a Claisen/Diels-Alder reaction cascade that, in combination with a Pd(0)-catalyzed reverse prenylation, provides rapid and efficient access to the CGX pharmacophore, represented by the structure of cluvenone. To further explore this pharmacophore, we have synthesized various A-ring oxygenated analogues of cluvenone and have evaluated their bioactivities in terms of growth inhibition, mitochondrial fragmentation, induction of mitochondrial-dependent cell death and Hsp90 client inhibition. We found that installation of an oxygen functionality at various positions of the A-ring influences significantly both the site-selectivity of the Claisen/Diels-Alder reaction and the bioactivity of these compounds, due to remote electronic effects.